Power steering systems, which were employed in approximately 20% of all passenger cars about five years ago, have recently become so popular that approximately 80% of all passenger cars employ a power steering system. Accordingly, the type of the hydraulic fluid for power steering is changing from the double-purpose type which also employs automatic transmission fluid (AFT) to the single-purpose type which is an oil for power steering.
In use, a hydraulic fluid for power steering is circulated by means of a vane pump or a gear pump at a pressure of from about 80 to about 105 kg/cm.sup.2 G at a temperature of about 60.degree. to about 120.degree. C. Moreover, the hydraulic fluid is normally not replaced before the car is scrapped. Moreover, the hydraulic fluid is required to have low temperature starting characteristics and to provide smooth cylinder movement in cold areas. Therefore, the hydraulic fluid should prevent abrasion, should show low temperature fluidability, have good thermal oxidation stability, have low friction characteristics and be inert to sealing materials used in the power steering system. To satisfy these requirements, various additives are incorporated in a selected base oil to provide a hydraulic fluid composition for power steering.